The present invention relates generally to Invar tool fabrication. More particularly, the invention relates to a method for fabricating an Invar tool including the step of superplastically forming an Invar facesheet to match or to approximate a predetermined contour of the tool.
Modern day aerospace, defense, and high tech research applications typically require complex, high precision, and close tolerance tooling. Thermal expansion is an area of particular concern in the aerospace industry with regard to tooling. For example, most military aircraft applications stipulate that tools exhibit minimal dimensional changes when subjected to extreme temperature variations.
In the campaign to control thermal expansion, a useful material has been developed under the tradename of Invar 36. Invar 36 (xe2x80x9cInvarxe2x80x9d) is a nickel-iron alloy that has a rate of thermal expansion that is approximately one-tenth that of carbon steel at temperatures up to 400 degrees F. Invar therefore has a particular attractiveness in fabricating tool components, including a header, which serves as a xe2x80x9cframexe2x80x9d for a tool, and a facesheet, which is welded to the header.
Several of our first approaches in developing the present invention were labor and material intensive. One such approach involved a stretch forming technique as shown in FIGS. 3 and 4. Specifically, a stretch form die was fabricated to a predetermined contour, and the Invar facesheet was stretched over the die surface. We considered this forming process to be unsuccessful as it required a substantial amount of excess Invar material for the jaws of the stretch press which are used to grip the facesheet. As the amount of this excess Invar material was typically equal to the amount of material that was required to make the facesheet, and as the excess material could not be used as the part of the tool, the excess Invar material needed to be trimmed, which led to substantial waste. Another one of our approaches involved bump forming thick Invar material, and cutting the material to fit the tool in segments. Yet another one of our approaches included the use of castings. This approach, however, was very expensive due to the need to eliminate porosity. It is therefore desirable to provide a method and assembly for fabricating an Invar tool with reduced labor and material costs.
A preferred method of the present invention for fabricating an Invar tool includes the step of providing a tool header defining a predetermined contour. An Invar facesheet is then superplastically formed to match, accurately or substantially, the predetermined contour of the tool header. Normally, the method further provides for welding the Invar facesheet to the tool header. Forming the Invar facesheet in this way results in less scrap, and therefore substantially reduces material costs. Forming the Invar facesheet in this way also leads to reduced labor costs.
The present invention further provides an Invar tool. The Invar tool includes a tool header defining a predetermined contour, and one or more Invar facesheets welded to the tool header. The Invar facesheets are formed with an SPF diaphragm driver to substantially match the predetermined contour of the tool header.
Further in accordance with the present invention, a method for molding parts on Invar tooling is provided. The method includes the step of making an Invar tooling surface from a sheet of Invar with SPF diaphragm forming. The sheet is placed on a tool header, and a part is formed against or on the tooling surface.